1. Field of the Invention
The present invention relates to a toner for developing electrostatic images used in electrophotography etc., and a positively chargeable charge control agent used in said toner for developing electrostatic images etc.
2. Description of the Prior Art
In copying machines, printers and other devices based on electrophotography, various dry or wet toners comprising a coloring agent, a fixing resin and other substances are used to visualize the electrostatic latent image formed on the photoreceptor having a light-sensitive layer containing an organic or inorganic photoconductive substance. There are two types of development processes using dry toners: methods using two-component developers comprising a mixture of a toner and a carrier, and methods using one-component developers comprising a toner used alone without being mixed with a carrier.
Development processes using two-component developers include those wherein a toner and a carrier are mixed and subjected to friction to mutually oppositely charge them, after which the charged toner visualizes an oppositely charged electrostatic latent image; specifically magnetic brush development, cascade development, etc. are used according to the kinds of toner and carrier.
One-component development processes include the powder cloud method, wherein toner particles are used in the form of a spray, the impression development process, wherein toner particles are brought into direct contact with the surface of an electrostatic latent image, and the inductive development process, wherein a magnetic electroconductive toner is brought into contact with the surface of an electrostatic latent image.
The chargeability of toners used in various development processes as described above is a key factor in electrostatic latent image-developing systems. Thus, to appropriately control or stabilize toner chargeability, various charge control agents providing a positive or negative charge are added to the toner.
In recent years, as photoconductive materials for the formation of electrostatic latent images in developing systems such as copying machines and laser printers, the consumption of maintenance-free organic photoconductive photoreceptors of low cost and high sensitivity has increased dramatically in the industry. To develop the electrostatic latent image formed on such organic photoconductive photoreceptors, the use of a good positively-chargeable toner is desired. Also, when a conventional selenium photoreceptor is used, the use of a positively-chargeable toner is essential for reversal development.
Of the conventional charge control agents in actual application, those providing a positive charge include basic dyes such as nigrosine dyes and triarylmethane dyes. However, most charge control agents of dye structure are generally structurally complex and unstable; for example, they are likely to be decomposed or deteriorated to lose their initial charge control capability when exposed to mechanical friction or impact, temperature or humidity changes, electric impact, light irradiation, etc. Also, because dyes are chromatic, they lack versatile applicability to color toners, a recently strongly demanded property.
Among positively chargeable charge control agents capable of resolving these problems are the compounds having a nitrogen atom cation in their molecular structure, such as quaternary ammonium salts, iminium salts and pyridinium salts, described in Japanese Patent Unexamined Publication Nos. 119364/1982, 98742/1983, 1162/1991, 100491/1993 and 11904/1994, and the p-halophenylcarboxylic acid described in Japanese Patent Unexamined Publication No. 186752/1983.
Although the positively chargeable charge control agents mentioned here are mostly light-colored and advantageous in that they can be used in color toners, some are unsatisfactory in terms of thermal stability, environmental stability, resin dispersion uniformity, or charge control characteristics, thus calling for further investigation.
The object of the present invention is to provide a positively chargeable charge control agent that has as an active ingredient a metal complex salt or metal complex of new stable chemical structure showing excellent positive charge control characteristics, that is excellent in thermal stability and durability (charge control or enhancement characteristic stability in multiple repeated use), that does not adversely affect toner fixability and offset characteristic when used in a toner, and that is ideal for use in color toners; and a toner for developing electrostatic images that contains said metal complex salt or metal complex as a charge control agent.